The base
of the zone is defined by the evolutionary transition from Spongaster
berminghami to Spongaster
pentas and is coincident with the upper limit of the Stichocorys
peregrina Zone.

* In the
tropical Indian Ocean, Johnson et al. (1989) used the upper limits of Phormostichoartus
fistula and Phormostichoartus
doliolum to divide this zone into three zones, Stichocorys
peregrina (NR8), Phormostichoartus
fistula (NR9) and Phormostichoartus
doliolum (NR10).